The invention relates to a transparent glazing unit with a resistive heating coating.
The invention relates more particularly to a glazing unit whose resistive heating coating is a coating placed on a substrate and having thermal insulation and/or solar protection capabilities. The glazing units incorporating this type of coating, when they are intended for equipping vehicles, make it possible in particular to reduce the air-conditioning load and/or reduce excessive overheating (“solar control” glazing) and/or reduce the amount of energy dissipated to the outside (“low-e” or “low-emissivity” glazing) brought about by the ever growing use of glazed surfaces in vehicle passenger compartments.
One type of multilayer known for giving substrates such properties consists of at least two metal layers, such as a silver-based layer, each being placed between two coatings made of a dielectric. This multilayer is generally obtained by a succession of deposition operations carried out using a vacuum technique, such as cathode sputtering, optionally magnetically enhanced or magnetron cathode sputtering. Two very thin metal layers, called “barrier layers” may also be provided, these being placed beneath, on, or to each side of each silver layer, the underlayer as a tie, nucleation and/or protection layer, for protection during an optional heat treatment subsequent to deposition, and the overlayer as a protective or “sacrificial” layer so as to prevent impairment of the silver if the oxide layer that surmounts it is deposited by sputtering in the presence of oxygen and/or if the multilayer undergoes a heat treatment subsequent to deposition.
In particular for vehicle windshields, there is high demand in the market for heating versions, in which the heating means must by nature be the least visible, or least obstructing for viewing, as possible. Consequently, there is an increasing demand for a transparent heating coating for such glazing.
A general problem with heating coatings having a low light absorption is their relatively high surface resistance, which requires a high supply voltage, in any case for heated glazing of large dimensions or for long current paths, which voltage is in any case higher than the usual voltages on board vehicles. If it is desired to lower the surface resistance, this is accompanied, in the multilayer systems known hitherto, with a reduction in visible light transmission because the conducting layers have to be thicker.
For these technical reasons, glazing units heated by wires, which may be supplied without any problem by the on board voltage, are still preferably being mounted at the present time. These laminated glazing units with integrated heating fields in the form of very fine wires are, however, not accepted by all purchasers.
Patent DE 1 256 812 B1 discloses a glass pane that can be heated by a coating made of metal or a metal oxide deposited continuously on one of its surfaces. This publication aims to solve problems due to the high ohmic resistance of said coating, which is of the order of 200 Ω/□. However, to be able to heat this coating using a relatively low voltage from two lateral busbars, narrow printed electrodes of low ohmic resistance (called “auxiliary electrodes”) that extend from said busbars over the heating field are provided. Said auxiliary electrodes terminate only a short distance in front of the opposite busbar, and they overlap each other with an alternating polarity.
However, said lines, which are optically perceivable as a hatching, obstruct vision and detract from the optical appearance of the main viewing field of the pane thus produced. It is not possible to make use of the optical advantage of a transparent heating coating. It is for this reason that such a pane is designed only for a rear window of automobiles.
Another problem with heating coatings may arise owing to the fact that they are sometimes not able to be deposited uniformly over the entire surface of the transparent glazing, but one or more interruptions, called “communication windows”, have to be provided therein, which disturb the flow of the heating current and may form “hot spots” (local overheating) along the edges of this or these communication windows. Such communication windows serve to make the coating, which by nature is reflective for short-wave radiation, respectively infrared radiation, locally more permeable to certain data streams or signals.
To inject and extract the heating current in these coatings, at least one pair of electrodes (in the form of bands) or of busbars, which have to inject the current into the heating coating and distribute it over a wide front as uniformly as possible, is provided. In vehicle glazing, which is substantially more wide than high, the busbars are usually found along the longer edges of the glazing (in the mounted position, the upper and lower edges), so that the heating current can travel along the shortest path over the height of the glazing. At the same time, the aforementioned communication windows are most of the time located at the upper edge of the glazing and extend there over several centimeters of width.
Document WO 00/72635 A1 discloses a transparent substrate with a coating that reflects IR rays and a communication window produced locally by removal or omission of the coating.
Obviously, each communication window which modifies the uniformity of the coating disturbs the current flow. Local temperature spots (“hot spots”) appear, which may result in damage to the substrate (thermal stresses) and to the coating itself. This is not only the case when the coating is defective over a large area, but also when the communication window is formed by a relatively large number of slots that do not communicate with one another. These also result, in the surface part in question, in an appreciable increase in the layer resistance and also give rise to the abovementioned hot spots.
The last document mentioned proposes, as a means of reducing the problematic effect of an extensive communication window, to provide, along its edge, an electrically conducting band that has an ohmic resistance per square which is significantly lower than that of the heating layer. Said band purports to take the current around the cut. Preferably, a communication window is framed entirely by such a band. The band may be produced by printing a silver-containing conductive screen-printing paste and by baking it. However, it may also be applied by the deposition of an electrically conducting lacquer or by depositing a metal strip. In all cases, a conducting electrical connection of the band to the coating is of course necessary in order for it to operate.
The band may be concealed from view by superposing an electrically nonconducting opaque masking strip, for example made of black enamel. As a general rule, such masking strips are made up from black-colored nonconducting material (screen-printing paste) to be baked. Infrared radiation is not reflected by this material, but absorbed.
Document WO 03/024155 A2 discloses transparent glazing of this type with a heating coating, in which, on the one hand, a maximum nominal voltage of 42 V is indicated, which however aims also to solve the problem of “hot spots” along the edges of a communication window. In general, various voltage levels are used, a lower voltage being applied to shortened current paths (for example because of the communication window) so as to avoid local overheating. Specifically, the communication window region is cut out from the heating surface by placing a separate busbar between the communication window and the busbar located on the opposite side.
Also known, from document DE 36 44 297 A1, are many examples of heating coatings for a vehicle windshield that are divided. The divisions may thus be produced by parts that are not provided with surface layers and/or by notches produced mechanically or by a laser beam. They are used for suitably adjusting and deflecting a current flow within the coated surface and have to ensure as uniform as possible a current density in the surfaces in question.
Document WO 2004/032569 A2 discloses another configuration of transparent glazing with a heating coating, which also aims to achieve uniformity of the heating power in the surface by separating lines traced in the coating.
Document DE 29 36 398 A1 relates to measures intended for preventing current spikes in the transition between the busbars and the coating, in transparent glazing with a heating coating. In general, the aim is to reduce the sudden difference in resistance between the coating and the busbars using materials or shapes with a higher resistance for the latter, or else with intermediate resistances. The above document indicates surface resistances of the coating of between 1 and 10 ohms per unit area. In one of the many embodiments described in that document, the edge of each busbar turned toward the opposite busbar is of corrugated form. The formation of sharp points turned toward the heating coating must thus be avoided. This approach aims to appreciably lengthen the transition line between the busbar and the coating and consequently to reduce the current density in this transition. However, all these measures seem poorly suited to be able to supply the heating layer with a relatively low voltage.
It is also known to provide, on the incident face of photovoltaic solar cells, grid or comb electrodes (see for example document WO 03/075351 A1). They are often produced by screen printing and made up of a busbar placed along the edge of the solar cell and of a plurality of small comb teeth that extend from the busbar over the surface of the solar cell. These electrodes allow surface connection for the photovoltaic voltage, which is present on both faces of the absorber, between the front comb electrode and the rear metal electrode, respectively, over its entire surface without excessively reducing the penetration of light into the absorber.
Document DE 197 02 448 A1 discloses a heated mirror, on the glass substrate of which two comb-shaped conducting tracks or electrodes are placed, these being indented one in the other, with a PTC coating (i.e. one having a positive temperature coefficient of resistance) that covers them and fills the intermediate spaces between the comb teeth. However, that document does not consider the problem of making the heating invisible to the eye, because the conducting tracks and the heating layer may be placed behind the mirror layer.
Document DE 198 32 228 A1 discloses vehicle glazing with an electrically conducting coating that is optically transparent and used as an antenna. Purely capacitive high-frequency radio signals are picked up from the antenna layer using a coupling electrode, which is made up of several fine wires connected together and placed parallel to one another at a certain distance apart that is large compared with their diameter, which wires extend from the edge into the viewing field of the glazing and terminate therein, without continuing. There is no galvanic coupling between the coating and these wires, because each time they lie in different planes from the laminated glazing.
The busbars already mentioned many times may be produced on the glass pane equally well by printing (screen printing) before or after deposition of the coating, or by soldering thin strips of sheet metal, preferably made of tinned copper. Combinations of printed busbars and metal-strip busbars are also known (see for example document DE 198 29 151 C1). Admittedly, the busbars are usually narrow and in the form of strips, but they are not transparent. For optical reasons, they are therefore placed each time near the outer edge of the transparent glazing units in question. Most of the time, they may be masked by opaque edge coatings (usually produced by screen printing). Likewise, the aforementioned communication windows may be masked by these edge coatings, provided that they are sufficiently permeable to the radiation to be transmitted via the communication window.
In standard vehicle windshields, these opaque coatings are in the form of a frame provided all around the glazing, which frame also has the function of protecting the bonded joint between the glazing and the body from UV radiation. These frames surround the general viewing field of the glazing. In windshields, a distinction may also be made between the main viewing field A, approximately in the middle of the area of the glazing, in which there can be no perceptible impairment of vision (for example by colorations or wires or other damage larger in size than 30 microns), and the secondary viewing field B closer to the edges.
The problem at the basis of the invention therefore consists in how to provide a transparent glazing unit provided with a heating coating that can operate with relatively low nominal voltages, in particular around to 14 volts, and which nevertheless produces a uniform distribution of the heating, in particular without any hot spot, with viewing in the general viewing field of the glazing, and in particular in the main viewing field A of the glazing, which is impeded as little as possible.